The problem of providing antennas usable on the nose of high-speed fighter aircraft requires meeting antenna performance criteria, while also meeting constraints limiting size, height, pilot view obstruction, air resistance, available mounting space, overall complexity, etc. While in many cases prior art antenna designs are available to meet desired antenna performance criteria, typically such prior designs cannot meet the very real physical constraints imposed for applications on fighter aircraft. The present inventor's prior applications directed to "Array Antenna With Forced Excitation" (No. 07/458,220, now U.S. Pat. No. 5,206,656) and to "Aircraft Antenna With Coning and Banking Correction" (No. 07/841,901, now U.S. Pat. No. 5,214,436) respectively relate to linear array antennas in which efficient broadband operation is achieved through forced excitation of three or more small radiating elements, and to antennas using a parallel array of such forced-fed antennas or other antennas.
In attempting to design two-element end-fire arrays for applications subject to such constraints, it was found that antennas using relatively large radiating elements could be provided. However, no solution permitting use of small elements while maintaining desired antenna performance over a significant operating band of frequencies was available. With small elements used in an end-fire array of monopoles, for example, the rear element has unusually low radiation resistance because of effects of mutual coupling which are severe with the small elements. This low radiation resistance increases the Q of the rear monopole, resulting in a poor impedance match over an operating frequency band.
In order to lower the Q of the rear element in such a two-element end-fire array, the height of the monopole could be increased or loss, i.e., series resistance, could be inserted. Both of these approaches are undesirable, particularly in the applications in point. For a three-element end-fire array, a solution was provided in the referenced prior applications by effectively offsetting the low radiation resistance of the rear element with the high radiation resistance of the forward element by use of a forced excitation system. That solution was effective in the three element array because the rear and forward elements are excited with signals of opposite phase. However, in a two-element end-fire array the elements are excited in quadrature phase, which precludes use of the forced excitation system.
It is therefore an object of this invention to provide improved dual-element end-fire array antennas suitable for aircraft applications, particularly those subject to size, height and other constraints.
Further objects are to provide new and improved end-fire linear array antennas utilizing small radiating elements and employing a special Q equalization circuit connected between the radiating elements, and antenna systems incorporating such linear array antennas.